EP1021843B1 - Electromechanical converter and method for producing the same - Google Patents

Abstract

The present invention relates to an electromechanical converter, a method of manufacturing same, as well as to a composite system including the electromechanical converter. The inventive electromechanical converter consists of at least one elongate fiber (1) made of a piezoelectric material. At least two electrodes (2) are provided at mutually spaced locations on the fiber (1) for subjecting the fiber to an electrical field. The electrodes (2) are in direct contact with the fiber (1) and enclose the fiber (1) partly or completely. The direct contact between the electrode and the fiber permits the optimum introduction of the electrical field into the fiber such that lower electrical potentials on the electrodes are sufficient for polarising the fiber. Another advantage resides in the aspect that the electrode is in direct contact with the fiber not only at one point but over part of the periphery of the fiber. This results in a further enhancement of the introduction of the electrical field.

Description

The invention relates to an electromechanical Transducer, method of manufacturing the same and a Compound system that includes the electromechanical transducer contains.

A preferred field of application of the invention is the area of adaptive materials. Active materials changing environmental conditions can be reversible and defined adjust when using an adaptive Circuit can be controlled. For example, they enable Vibration damping, noise reduction and contour control of components. There is a special one here Need for mechanically resilient composite materials, which contain sensory and actuator components.

Electromechanical transducers, how they work based on the piezoelectric effect are suitable in special way for the integration or integration in such composites. These converters work as sensitive force or displacement sensor and show as Actuator an outstanding potential for power or Path generation with high dynamics.

Ideally, the electromechanical transducer in this way into the overall structure of a composite material can be integrated that the mechanical properties this structure is affected as little as possible become. Furthermore, the converter should be used with one low electrical voltage can be.

The goal of this structurally compliant integration, i.e. the minimal disturbance of the forest especially through the use of fibrous piezoelectric materials, especially ferroelectric ceramics, can be achieved.

In R.E. Newnham, D.P. Skinner, L.E. Cross, Mat. Res. Bull., 13 (1978) 525, is the conception and production of composite materials or composites from functional ceramics and a polymer matrix described. Among other things, the advantages of composites, the fibers or rods made of ferroelectric ceramic included, calculated and verified (see e.g. R.E. Newnham, A. Safari, J. Giniewicz, B.H. Fox, Ferroelectrics, 60 (1984) 15-21). In this work the composites consist of thin slices of approx. 1 mm thick, in which the fibers are perpendicular to the face are arranged. The fibers are through Electrode the end faces polarized and controlled.

Ferroelectrics need for their polarity, which for It is necessary to achieve the piezoelectric properties is electric fields of a certain size. The Polarity takes place between two electrodes with the distance d and with an electrical voltage U. The resulting electric field E = U / d increases with increasing Distance d of the electrodes. Therefore, longer ones Fibers are not polarized by electrodizing their ends as the required polarity fields from to at 6 kV / mm from fiber lengths of a few mm in the Practice cannot be achieved. The polarization long fibers, i.e. one or more cm long, in the longitudinal direction is therefore in a conventional discontinuous poling step with two electrodes not possible on the end faces because the required electrical fields require voltages of over 100 kV would.

In A.A. Bent, "Active Fiber Composites for Structural Actuation ", Ph.D. Thesis, MIT, January 1997, describes a technique with which longer fibers in Longitudinal direction can be polarized. To do this at certain intervals along the longitudinal axis of the fibers Electrodes applied to the fibers in sections head for. The electrodes become positive alternately and connected negatively. Thus, the in the fibers electrical fields required for polarity by far lower electrical voltages can be achieved than when the fibers across the front faces as a whole Pole length and control would have to be. to The electrodes are initially manufactured using this system in structured form on Kapton layers applied. This is done by applying one Silver ink using the technique of so-called screen printing. The fibers are between these Kapton layers arranged that the adhesive bond to the fibers. Thus lies with the finished one Transducer an arrangement in which the fibers between two Kapton layers are embedded on which each has a flat electrode structure.

However, such an arrangement has the disadvantage that despite the realizable short distances between the individual electrodes are still relatively high Tensions are needed to the individual To polarize fiber areas.

The object of the present invention is therefore an electromechanical converter and method for Manufacture to provide the same for the Integration in a network system is suitable and the Polarity and control with lower electrical Tensions.

This task is done with the electromechanical Converter according to claim 1 and the method according to Claims 13 and 14 achieved. Advantageous configurations the invention are the subject of the dependent claims.

The electromechanical transducer according to the invention consists of at least one elongated fiber or a thin rod made of a piezoelectric Material. At spaced apart locations At least two electrodes are provided around the fiber To apply an electric field to the fiber. The Electrodes are in direct contact with the fiber Contact and at least partially enclose the fiber. In this context, direct contact also means that there is no other one between the electrode and the fiber, for example dielectric layer.

This direct contact between the electrode and fiber is the optimal coupling of the electrical Allows field in the fiber, so lower electrical voltages at the electrodes are sufficient, to polarize the fiber. Another The advantage of the construction according to the invention is that that the electrode is not only at one point with the Fiber is in direct contact, but over one Part of the perimeter of the fiber. This creates the coupling of the electric field further improved.

The electrodes preferably consist of a conductive adhesive. This can be done easily Apply wisely, adheres to the fiber, stabilizes this mechanically and leads in contacting the Fiber automatically to partially enclose the Fiber.

Another improvement in electromechanical Transducer is achieved in that the individual electrodes completely enclose the fiber. The electrodes form ring electrodes around the fiber.

The electromechanical transducer preferably has Fibers with a diameter of less than 100 µm and with a length of 5 to 100 mm.

In a preferred embodiment there are several of these fibers in parallel next to each other (monolayer) and are made of electrodes in interdigital geometry contacted, which run transverse to the longitudinal axis of the fibers.

An essential feature of the invention The converter consists in direct contact of the conductive Material with the fiber. This will make the required Voltage to generate a certain field strength already significantly reduced in the fiber. Furthermore, this results at least in part Enclose the fiber (with direct contact) further improving the coupling of the field strength and thus a reduction in the required electrical Tensions. Particularly advantageous for the production is the use of a conductive adhesive Formation of the electrodes.

In a preferred embodiment of the invention are long ferroelectric fibers to form a Monolayer arranged side by side in parallel. The fibers are polarized and controlled by the electrodes along their longitudinal axis at short intervals be applied, as described for example in A.A. Bent, "Active Fiber Composites for Structural Actuation ", Ph.D. Thesis, MIT, January 1997 becomes. These electrodes are alternately positive and connected negatively. Thus, the in the fibers electrical fields required for polarity low electrical voltages can be achieved.

The concept of electroding according to the invention is preferably made by parallel tracks of conductive Realized glue perpendicular to the Alignment of the monolayer of parallel fibers. For the provision of large converter areas several transducers constructed according to the invention side by side to be ordered.

For the integration of the converter according to the invention with the piezo fibers in a composite laminate, are above and below the active single layer laminated further layers of the composite material.

With such a composite material opens up a number of fields of application in the most varied Industries. Damage to composite structures can be captured using the direct piezo effect without the sensors, i.e. the or the electromechanical transducers according to the invention, the Structure of the composite is disturbed. By the inverse piezo effect, the transducers can be as active Composites the geometry of composite parts change. A mechanical vibration can Composites active by generating a counter wave or passively by short-circuiting those generated with the piezo effect Dampen tensions.

Here, further improvements can be made through the The following measures can be achieved represent further embodiments of the invention.

By reducing the distance between the Electrodes, i.e. the comb or interdigital electrodes, to less than 100 µm they are used for polarization and control of the fibers required tensions further reduced.

For a structurally compliant integration of the electromechanical Transducers have the functional fibers a diameter well below 100 μm, preferably <30 µm. Only with such dimensions do they put in introduced a fiber composite material, a minimal structural disorder and can occur without the occurrence high mechanical moments can be bent.

To achieve structural conformity furthermore the fibers only in a very thin composite single layer, for example a polymer layer, embedded, which then in turn in the overall composite material can be integrated.

With the method according to the invention, composite layers can thinner than 200 µm, one electrode gap smaller than 100 µm and a parallel fiber placement of more than 100 fibers per cm achieved side by side become. When using ferroelectric ceramic fibers as functional fibers, these can be polarized forces and then smaller as a pressure sensor as 1 N by an alternating electric field Vibrations are excited and introduced from the outside mechanical vibrations active (in combination with an adaptive circuit) or passive damping.

As piezoelectric fibers or functional fibers PZT fibers are preferred. This Fibers can be processed by methods such as in described in DE 43 32 831 or DE 196 35 748, getting produced.

Of course, the invention is electromechanical transducers are not preferred Area of application of the composite materials limited. So can also be an electromechanical transducer, for example with a single fiber as an actuator or sensor microtechnology.

Figur 1

schematisch ein Beispiel für einen erfindungsgemäßen Wandler;

Figur 2a

ein Beispiel für die Elektrodierung eines Bereiches einer einzelnen Faser gemäß der Erfindung;

Figur 2b

ein Beispiel für die Aufbringung einer weiteren Schicht auf die Faser der Figur 2a; und

Figur 3

schematisch die Geometrie einer Elektrodenstruktur in einer Ausführungsform der Erfindung.

The present invention is explained in more detail below on the basis of the exemplary embodiments in conjunction with the drawings. Show here

Figure 1

schematically an example of a converter according to the invention;

Figure 2a

an example of the electroding of a region of a single fiber according to the invention;

Figure 2b

an example of the application of a further layer on the fiber of Figure 2a; and

Figure 3

schematically the geometry of an electrode structure in one embodiment of the invention.

A process for the production of composite materials, the electromechanical according to the invention Transducer with controllable piezoelectric fibers, PZT fibers, for example, are subdivided in the manufacture and application of the electrode structure, in the production of fiber fabrics, in the Placing the fibers on the electrodes or electrodes on the fibers, in the pouring and laminating this structural element and the subsequent poling of the Fibers in the structure.

Figure 1 schematically shows an example of a converter according to the invention. In this example there are five Fibers (1) parallel to each other on a base (3) arranged. The fibers are along their long axis at spaced apart locations by electrodes contacted, arranged in interdigital geometry are. The electrodes here have direct Contact the fibers and enclose the individual Fibers partially, as can be clearly seen from the figure is. The electrical connection of the arrangement is made over the two pads (4) of the electrodes.

Figure 2a shows an example of the electrodeposition an area of a single fiber (1). In this The electrode structure (2) consists of an example first layer or layer (2a) and an overlying one second layer (2b). The second layer is here optionally provided. While the fiber in the first Layer is partially embedded, so that the fiber of half of the electrode is made possible the second layer completely enclosing the fiber. In this way, a ring electrode is formed which encloses the fiber.

An example of a method for producing the Electromechanical transducer according to the invention is in following explained in connection with Figure 2. The Structure of the lower electrodes (2a), for example an interdigital structure as shown in Figure 3 placed in a thin template as a negative form. With a squeegee step with conductive adhesive the electrode tracks (2) over the negative form squeegee on a support (3). The document can consist for example of a carrier layer, or from the composite itself, into which the converter should be integrated.

A is preferred as the conductive adhesive thixotropic epoxy resin that contains metal particles (e.g. from Ag) contains used. This adhesive can be on the one hand, lightly into the negative template under pressure delete and flows on the other hand when the Not stencil while keeping straight sides further. Before applying the fibers (1) to them The structure of the adhesive can be hardened to such an extent that it does not flow even under pressure, but still the Necessary stickiness that fibers on it be liable. The hardening is done to a To prevent the adhesive from entering the capillaries, between the fibers and the base in the further course can result. It takes place preferably at elevated temperatures. The fibers (1) are then applied to the electrodes (2a). The electrodes are then fully cured. The structure can now be cast, for example by applying a resin (5).

An improvement in the properties of the converter is achieved if additionally before shedding a further electrode layer (2b) on the fibers (1) is applied, which is largely congruent with the first electrode layer (2a). This structure will with the help of a dispenser, the conductive adhesive can process, distributed over the fibers so regularly, that these are at the crossing points of the electrode tracks with the fibers completely from the electrode be surrounded. In this case, a is preferred conductive adhesive of lower viscosity than that for the lower electrode layer (2a) is used. To This adhesive is also applied to the structure hardened. Then this structure can also as shown in Figure 2b, with a resin (5) be shed.

In another exemplary method for Production of an embodiment of the invention The fibers are first converted in parallel Poured assembly into a polymer. following is the structure for the electrodes with a laser (2) locally removed in the polymer layer until the The fiber surface is exposed. The wavelength and energy the laser should be set so that the plastic is removed without damaging the fibers. To the one-sided introduction of the structure becomes the electrode arrangement made by filling the wells. A conductive adhesive is preferred used. Of course, other materials are also possible, for example by steaming the fibers with a metal. The electrodes are the fibers each completely enclose the steps Laser ablation and filling for the opposite side the polymer layer is repeated.

The preferably ferroelectric fibers used for a flat composite material are used for example PZT fibers, as described in DE 43 32 831 or DE 196 35 748 are described. These will about a wet chemical process, a sol-gel process, manufactured and via thermal steps in ceramic Fibers converted.

The fibers are preferably electrodeposited as a monolayer, i.e. they are arranged in parallel without Fibers come to lie on top of each other. To achieve this, the fibers are either mono or Multifilament fiber (see DE 196 35 748) manufactured and then placed in a monolayer, or she are already produced as a monolayer.

Another option is a monolayer of PZT fibers is to get the fibers already through the spinning process, in which it is still called the sol-gel stage are present in a group of parallel fibers bring that largely do not overlap or cross. In the subsequent drying steps, The fibers may need pyrolysis and sintering thermally treated on various substrates what makes it necessary to fold the fibers. Since the most functional fibers should be very fragile a gentle process is used here the fibers without disturbing their parallel arrangement, on another surface. For example using a vacuum suction technique.

The application of the fibers to the electrode structure or the electrode structure on the fibers possible with different methods.

A carrier layer can be made with the electrodes still wet conductive adhesive on the fiber fabric be lowered. With some pressure on the The fibers are then inserted into the electrode structure Glue pressed.

Another possibility is the successive Applying individual fibers to the electrode structure This can be done manually or in that the fibers pass through a slot from above, which runs perpendicular to the electrode tracks these fall down. By moving the slot vertically the fibers become the electrode tracks one behind the other and filed parallel to each other.

For integration into a composite material the one-sided or all around electrodized fibers preferably potted with a polymer. Is preferred for this an epoxy resin is used, that of the matrix material of the composite material is largely the same.

This layer can be combined with any number of others Single layers of laminate, which in turn are PZT fiber structures may contain, overlaminated.

The fibers were like the second one described above Process after embedding in a polymer Electrodized, the polymer layer with the fibers can be laminated directly into the composite.

At the outer connections (4), when pouring have not been covered with polymer Transducer by control electronics or measuring circuit a voltage is applied by which the fibers are preferred at elevated temperatures, polarized in the structure become. Via the connections (4) the fibers for their sensory or actuator tasks are also controlled.

With multiple arrangement of the invention Transducer within the composite structure, e.g. side by side or one above the other, can be delayed by a phase Actuation of the transducers targeted vibrations be stimulated.

Claims (19)

1. An electromechanical transducer with at least one elongated fibre (1) of a piezoelectric material and at least two electrodes (2) which are provided at mutually spaced places on the fibre, in order to impose an electric field in the longitudinal direction on the fibre, characterized in that the electrodes (1) are in direct contact with the fibre (1), the individual electrodes at least partially surrounding the fibre.
2. An electromechanical transducer according to claim 1, characterized in that the electrodes (2) are formed from an electrically conductive adhesive.
3. An electromechanical transducer according to claim 1 or 2, characterized in that the individual electrodes (2) completely surround the fibre (1).
4. An electromechanical transducer according to any of claims 1 to 3, characterized in that the fibre (1) has a diameter of less than 100 µm.
5. An electromechanical transducer according to any of claims 1 to 4, characterized in that the fibre (1) has a length from 5 to 100 mm.
6. An electromechanical transducer according to any of claims 1 to 5, characterized in that a plurality of electrodes (2) are arranged along the longitudinal axis of the fibre (1) in an interdigitatated geometry.
7. An electromechanical transducer according to any of claims 1 to 6, characterized in that a plurality of fibres (1) are arranged parallel alongside one another, so that a mono-layer of fibres is formed.
8. An electromechanical transducer according to claim 7, characterized in that a plurality of mono-layers of the fibres (1) lie one on the other.
9. An electromechanical transducer according to claim 7 or 8, characterized in that further layers (5) of other materials are attached to the mono-layer(s), on top and/or underneath.
10. An electromechanical transducer according to any of claims 1 to 9, characterized in that the fibres (1) are embedded in a polymer layer.
11. An electromechanical transducer according to any of claims 1 to 10, characterized in that the piezoelectric material is a ferroelectric ceramic.
12. A composite material of a plurality of layers, characterized in that at least one of the layers comprises the electromechanical transducer according to any of claims 1 to 11.
13. A method of making an electromechanical transducer with the following steps:

    providing one or more elongated fibres (1) of a piezoelectric material;

    applying the fibre(s) (1) to a supporting layer (3); and

    applying electrically conductive material for forming electrodes (2) at at least two places spaced from one another, directly on the fibre(s), so that the conductive material is in direct contact with the fibre(s), at least partially surrounds the fibre(s), and an adhesive connection between the fibre(s) and the conductive material is created.
14. A method of producing an electromechanical transducer with the following steps:

    providing one or more elongated fibres (1) of a piezoelectric material;

    applying electrically conductive material for forming electrodes (2) at at least two places spaced from one another on a supporting layer (3),

    applying the fibre(s) (1) directly on the conductive material, so that the conductive material is in direct contact with the fibre(s), at least partially surrounds the fibre(s), and an adhesive connection between the fibre(s) and the conductive material is created.
15. A method according to claim 14, characterized in that conductive material is further applied directly on the fibre(s) at places whereat which the fibre or fibres is or are already partially enclosed by conductive material, so that the conductive material completely surrounds the fibre(s).
16. A method according to claim 14 or 15, characterized in that a negative pattern for definition of an interdigitated geometry of the electrodes is applied on the supporting layer before the application of the conductive material, whereby the conductive material is applied in the negative pattern.
17. A method according to any of claims 13 to 16, characterized in that the conductive material is an electrically conductive adhesive, especially a thixotropic polymer.
18. A method according to any of claims 13 to 17, characterized in that the supporting layer (3) is put down on the fibres with still wet conductive adhesive, whereby the fibres are pressed into the adhesive.
19. A method according to any of claims 13 to 17, characterized in that the supporting layer (3) is part of a composite material in which the electromechanical transducer is to be integrated.

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